A simple source inversion scheme for displacement seismograms recorded at short distances

We have implemented and tested a simple inversionscheme to retrieve source parameters of small andmoderate earthquakes recorded at close distances. Theinversion assumes that such events may be approximatedby a point-source shear dislocation, and the mediummay be considered an infinite space. Theoreticalseismograms used in the inversion include near- andintermediate-field contributions. The effect of freesurface is approximately taken into account bymultiplying the infinite-space synthetics by two. Thelocation of the event is assumed to be known. Based onan examination of the data, the user chooses thelength of the record and the weight of each trace tobe used in the inversion. The shape of the source-timefunction is estimated from the observed seismograms.A grid search is performed to determine the focalmechanism and the seismic moment which minimizes theerror between the observed and the syntheticseismograms. For two or more recordings, the inversiongives a solution which is consistent with thefirst-motion data and/or the moment tensor inversionof regional seismic waves. If only a singlethree-component recording is available then theinversion yields more than one equally acceptablefocal mechanisms. In such cases, the first-motionpolarities or a prior knowledge of the tectonics ofthe region is essential to constrain the solution. Thetest of the method on near-source broadband recordingsof small and moderate earthquakes in Mexico and theaftershock sequence of 1997 Umbria-Marche, Italy showsthat it gives fast, and surprisingly robust andreliable estimation of source parameters.     

Seismic behavior assessment for design of integral abutment bridges in Illinois

Integral abutment bridges(IABs)minimize deterioration and degradation of the abutment seats and bearings due to water,dirt,and deicing chemicals by eliminating ...     

Use of rubberised backfills for improving the seismic response of integral abutment bridges

Reuse of the 1.5 billion waste tyres that are produced annually is a one of the major worldwide challenges, as waste tyres are toxic and cause pollution to the environment. In recognition of this problem, this paper introduces the reuse of tyres, in the form of derived aggregates in mixtures with granulated soil materials, as previous studies indicated the potential benefits of these materials in the seismic performance of structures. The objective of the present research study is to investigate whether use of rubberised backfills benefits the seismic response of Integral Abutment Bridges (IABs) by enhancing soil-structure interaction (SSI) effects. Numerical models including typical integral abutments on surface foundation with nonlinear conventional backfill material and its alternative form as soil-rubber mixtures are analysed and their response parameters are compared. The research is conducted on the basis of parametric analysis, which aims to evaluate the influence of different rubber-soil mixtures on the dynamic response of the abutment-backfill system under various seismic excitations, accounting for dynamic soil-abutment interaction. The results provide evidence that the use of rubberised backfill leads to reductions in the backfill settlements, the horizontal displacements of the bridge deck, the residual horizontal displacements of the top of the abutment and the pressures acting on the abutment, up to 55, 18, 43 and 47 % respectively, with respect to a conventional backfill comprising of clean sand. Small change in bending moments and shear forces on the abutment wall is also observed. Therefore, rubberised backfills offer promising solution to mitigate the earthquake risk, towards economic design with minimal damage objectives for the resilience of transportation networks.     

液体黏滞阻尼器在大跨度桥梁中对伸缩缝保护作用的探讨

为了限制动荷载作用下梁端的位移,经常要在梁端安装阻尼器,与伸缩缝在结构上形成并联装置。当桥面突然发生大规模的刹车,或在突加的地震、风荷载等荷载的冲击作用下,阻尼器是否会对伸缩缝起到保护作用就成为一个令人关心的问题。由一般多自由度体系动力方程出发,通过矩阵变换得到结构局部关于阻尼器单自由度方程。基于此单自由度方程,对阻尼器在结构中具有局部减震效果提出了理论上的论证,在此基础上就阻尼器在各种冲击荷载作用下对伸缩缝的保护作用进行了探讨。     

Optimization of the seismic response of bridges using variable-width joints

An aspect of seismic design of bridges that has hardly received proper attention so far is the appropriate selection of joint gaps. End gaps define the boundary conditions of the bridge and affect its dynamic response; their proper design can lead to an improved structural performance under dynamic actions. The idea of the ‘Dynamic Intelligent Bridge’ is explored here, wherein current bridge joints that have a fixed width are substituted by variable-width joints and, under seismic loading, the joint gap is optimised either with a one-off adjustment, or continuously (in real time) through semi-active control. In all cases, a novel device is used that permits this improved behaviour of the joints, the moveable shear key (MSK), a device for blocking the movement of the bridge deck, which has the possibility to slide, hence varying the size of the existing joint gap. In this context, the effect of gap size on the seismic response of bridges is assessed herein and a methodology is put forward for optimising this size, using a number of criteria such as maintaining the functionality of the bridge for moderate earthquakes, and ensuring the safety of the bridge and its users under earthquakes stronger than that used for design.     

A simple model for structural control including soil–structure interaction effects

A simple model for the seismic response of a one-storey structure subjected to active control in the presence of soil–structure interaction effects is presented. The approach is based on the successive use of equivalent 1-DOF oscillators which account for the effects of control and soil–structure interaction. Simple expressions for these oscillators based on exact analytical solutions of the control equations and approximate solutions of the interaction equations are presented. The study includes an evaluation of the effects of soil–structure interaction on the seismic response of actively controlled structures in which the control gains have been determined with and without inclusion of soil–structure interaction effects. A simple procedure to include the interaction effects on the control gains is also presented. © 1998 John Wiley & Sons, Ltd.     

A simple model for the formation of vegetated dunes

A simple model for the dynamics of dunes associated with vegetation is proposed. Using the model, the formation processes of transverse dunes, parabolic dunes and elongated parabolic dunes are simulated according to two environmental factors: (i) the amount of sand at the source; (ii) the wind force. The results have qualitative correspondence to the real counterpart, and the simplicity of the algorithm and the consequent ease of handling this model provide us with wide applicability for the investigation of the complex interplay between vegetation and dunes. Copyright © 2001 John Wiley & Sons, Ltd.     

A non-linear numerical model of the tuned liquid damper

The Tuned Liquid Damper (TLD) is modelled numerically as an equivalent tuned mass damper with non-linear stiffness and damping. These parameters are derived from extensive experimental results described in References 1 and 2. This Non-linear Stiffness and Damping (NSD) model captures the behaviour of the TLD system adequately under a variety of loading conditions. In particular, the NSD model incorporates the stiffness hardening property of the TLD under large amplitude excitation. Copyright © 1999 John Wiley & Sons, Ltd.     

Assessment and modeling of embankment participation in the seismic response of integral abutment bridges

The dynamic response and seismic performance of bridges may be appreciably affected by numerous contributing factors, with soil–structure interaction being the dominant exogenous influence. The most familiar form is the so-called soil–pile interaction, but embankment–abutment interaction is also documented through field observations and analytical investigations, particularly evident in integral R.C. bridges. Recent studies have shown that this form of interaction may significantly alter the bridge response and should be taken into account during design and assessment, especially in the case of typical highway overcrossings that have abutments supported on earth embankments. In light of this emerging problem and in order to facilitate quantitative estimates of the interaction effects, the question of appropriate modeling and seismic assessment of R.C. integral bridges is the main object of the present paper. Based on already established procedures to account for soil–structure interaction, a new approach is proposed to model the contribution of the embankment, the bent and the abutments to the overall bridge response. Furthermore, the capacity curve of the entire bridge system is evaluated through the implementation of Incremental Dynamic Analysis (IDA), therefore allowing for seismic assessment of the complex superstructure–foundation system with well established displacement based procedures. Using as a benchmark case two typical instrumented U.S. highway bridges located in California, the proposed method is implemented and provided results from this analysis are correlated successfully with available field data. Results obtained from the analysis indicate excessive displacement demands for the entire bridge–embankment system owing to the embankment contribution and the soil degradation under increasing shear strains. Furthermore, seismic performance is strongly related to the central bent deformation capacity, with soil–pile interaction effects being of critical importance.     

A simple model for spatial rainfall fields

Spatial rainfall amounts accumulated over short to medium periods of time, say a few days, tend to have a probabilistic structure with very distinctive features. Some of these that are specially relevant for the purpose of spatial modeling are the presence of mixed sampling distributions, right skewed distributions conditional on rainfall occurrence, and a complex spatial association structure. The goal of this work is to construct a family for the bivariate distributions of spatial rainfall fields that incorporates these distinctive features. It is based on the separate modeling of spatial occurrence of rainfall and the spatial distribution of positive rainfalls. The main properties of the bivariate distributions are derived, and some properties of the random field realizations are illustrated through simulation. Some limitations of the proposed model are also discussed.     

A simple oblique dip model for geomagnetic micropulsations

It is pointed out that simple models adopted so far have tended to neglect the obliquity of the magnetic field lines entering the Earths surface. A simple alternative model is presented, in which the ambient field lines are straight, but enter wedge shaped boundaries at half a right-angle. The model is illustrated by assuming an axially symmetric, compressional, impulse type disturbance at the outer boundary, all other boundaries being assumed to be perfectly conducting. The numerical method used is checked from the instant the excitation ceases, by an analytical method. The first harmonic along field lines is found to be of noticeable size, but appears to be mainly due to coupling with the fundamental, and with the first harmonic across field lines.     

A bi-directional gap model for simulating the directional thermal radiance of row crops

Row crops are a kind of typical vegetation canopy between discrete canopy and continuous canopy. Kimes et al. studied the directional thermal radiation of row crops using the geometrical optical model, which simplified row structure as “box” and neglected the gap among foliage and did not consider the emissivity effects. In this work we take account of the gaps along illumination and viewing directions and propose a bi-direction gap model on the basis of the idea of gap probability of discrete vegetation canopy introduced by “Li-Strahler” and inter-correlation of continuous vegetation developed by Kuusk. It can be used to explain “hot spot” effects in thermal infrared region. The gap model has been validated by field experiment on winter wheat planted in shape of rows and results show that the gap model is better than Kimes’ model in describing the directionality of thermal infrared emission for row crops.     

Effect of a secondary channel on the non-linear tidal dynamics in a semi-enclosed channel: a simple model

Non-linear tidal dynamics are investigated in a network that consists of a semi-enclosed main channel and a secondary channel at an arbitrary position. The water motion, governed by the one-dimensional shallow water equations, is forced by an incoming tidal wave. Solutions are obtained with the method of characteristics. The overall aim is to quantify and understand the spatial structure of different tidal harmonics (the principal tide and its non-linear overtides) and of tidal asymmetry for both the vertical and the horizontal tide in the main channel for different locations of the secondary channel. This is of practical interest in the context of possible construction of secondary channels to reduce tidal range in estuaries. Moreover, tidal asymmetry is an important factor in driving net sediment transport. Analysis of the different tidal harmonics shows that their characteristics are similar to those obtained with an earlier linear model. In particular, amplitudes of the harmonics are reduced landward of the secondary channel if the latter is positioned less than a quarter wavelength of the respective tidal wave away from the landward boundary. Thus, the distortions of the tide due to the presence of the secondary channel are generated locally and afterwards propagate through the network. Tidal asymmetry is quantified by examining tidal range, flood-to-ebb ratio and the duration of the falling tide and the duration between maximum flood and maximum ebb. A spatial non-uniform reduction in tidal range is observed that shows very localised increase and decrease depending on the position of the secondary channel. The changes in the velocity characteristics induce changes in net sediment transport. It turns out that the direction of the peak current, derived from the flood-to-ebb ratio, is not sensitive to the position of the secondary channel, whereas the duration between flood and ebb can change from more to less than half the tidal cycle. However, the changes in the velocity asymmetries are confined to a small region.     

A thin-layer interface model for wave propagation through filled rock joints

The present study essentially employs a thin-layer interface model for filled rock joints to analyze wave propagation across the jointed rock masses. The thin-layer interface model treats the rough-surfaced joint and the filling material as a continuum medium with a finite thickness. The filling medium is sandwiched between the adjacent rock materials. By back analysis, the relation between the normal stress and the closure of the filled joint are derived, where the effect of joint deformation process on the wave propagation through the joint is analyzed. Analytical solutions and laboratory tests are compared to evaluate the validity of the thin-layer interface model for filled rock joints with linear and nonlinear mechanical properties. The advantages and the disadvantages of the present approach are also discussed.     

A simple model for non-vanishing rotational hysteresis in haematite

The model, applicable to fine-grained haematite, assumes that the magnetization of a grain is constrained to lie in the basal plane, within which there is a single easy axis. Following Stoner and Wohlfarth (1948), torque curves and rotational hysteresis are calculated, for various field strengths, as a function of the orientations of the basal plane and easy axis. The results show that, for certain orientations, rotational hysteresis may be expected to persist to very high field strengths. Further, the dependence of rotational hysteresis on orientation implies that rotational hysteresis is anisotropic and thus, that, for a distribution of grains, the anisotropy of resultant rotational hysteresis will reflect the degree of preferred orientation. It may therefore serve as a rock fabric parameter. Calculated models of resultant rotational hysteresis for assumed preferred orientation distributions compare favourably with measurements on a sample of Cambrian purple slate from North Wales.     

A simple engineering model for the seismic site response of alluvial valleys

The seismic site response of alluvial valleys with limited width is studied. The intent of this investigation is to integrate the seismological and engineering perspectives to gain physical insight into the dynamic behaviour of alluvial valleys. A simplified engineering model (frame model) is developed to predict the nonlinear seismic response of symmetrical valleys. The proposed model is one-dimensional and accounts for the limited width of the valley. The frame model identifies the significant vibration modes and their variation in the horizontal and vertical directions. Sensitivity analyses are performed on the valley response to evaluate the effect of the uncertainty in establishing the dynamic soil properties. Response results from the frame model are compared with those calculated using one-dimensional and two-dimensional finite-element models. The proposed frame model response shows good agreement with finite-element model results.     

A simple model of hillslope response for overland flow generation

This paper deals with the derivation of the hydrological response of a hillslope on the assumption of quick runoff by surface runoff generation. By using the simple non‐linear storage based model, first proposed by Horton, an analytical solution of the overland flow equations over a plane hillslope was derived. This solution establishes a generalization for different flow regimes of Horton's original solution, which is valid for the transitional flow regime only. The solution proposed was compared successfully with that of Horton and, for the turbulent flow regime, to the one derived from kinematic wave theory. This solution can be applied easily to both stationary and non‐stationary rainfall excess events. An analytical solution for the instantaneous response function (IRF) was also derived. Finally, simple expressions to compute peak and time to peak of IRF are proposed. Copyright © 2001 John Wiley & Sons, Ltd.     

大型渡槽的竖向地震效应分析

基于流体的位移有限元模式,借助于Ansys程序,考虑流体与结构之间的相互作用(作滑动边界处理),对3种型式的渡槽结构竖向地震效应进行了时程与反应谱分析。计算结果具有较好的精度,满足工程要求。本文的方法也可直接用于大型渡槽的水平地震效应分析。对于不同结构型式的渡槽,在七度抗震设防时,建议适当考虑竖向地震效应。